This project is designed to continue the study of the topography of steroid binding sites with affinity labeling steroids. So far, we have studied 20 beta-hydroxysteroid dehydrogenase. We have worked out an affinity chromatography method (in press) which yields good quantities of homogeneous 17 beta-hydroxysteroid dehydrogenase from human placenta and we will begin the study of the residues at its binding site too. The 20 beta-hydroxysteroid dehydrogenase will be studied with 2alpha-bromoacetoxy-progesterone which seeks amino acid residues that proximate the two position of the steroid on binding. We will also evaluate the previously reported histidine labeling with a nonhydrolyzeable affinity labeling steroid, 16 alpha-bromoacylprogesterone. The first steroid for study of the 17 beta-hydroxysteroid dehydrogenase is 16 alpha-bromoacetoxyestradiol. This compound has been synthesized, purified, and is now under elemental analysis.